Magnetic fields are often used in the production or testing of articles. For example, magnetic and magneto-optic heads, which are used to read and write data on disk drives, are generally tested while placed in a magnetic field. It is important to test such heads to ensure that a defective head is not installed within a disk drive. Moreover, to reduce costs and/or to increase throughput, it is desirable to test for defective heads early in the production cycle.
One type of tester used to ensure device performance and reliability early in the production cycle tests the magneto-resistive characteristics of heads while they are in wafer form, which includes thousands of magneto-resistive (MR) heads. Typically only a subset of the MR heads in a wafer is tested. Testing MR heads in wafer form requires a probe to contact one or more of the MR heads while a magnetic field is generated perpendicular to the particular MR head or heads under test. Moreover, in wafer form, the MR heads are vertical and therefore the required magnetic field must be applied parallel to the surface of the wafer. For optimal test results the precise amount of field applied to the MR heads under test should be known and should be repeatable under ongoing test operations. Conventional testers use fringe magnetic fields, which unfortunately produce a magnetic field that is only approximately parallel to the surface of the wafer in a very small area. Accordingly, the number of MR heads that can be tested simultaneously with such a tester is very limited.
Thus, it is desirable to improve the production of magnetic fields to produce fields that are plane with the surface of a wafer or other item under test.